Optical amplifier and a method of preventing emission therefrom of optical power exceeding a prescribed safety limit

ABSTRACT

An optical amplifier comprises an active fiber ( 1 ), a pump unit ( 2 ) spaced from the active fiber and adapted to give a nominal, continuous pump power in an operational state, and a pump fiber ( 3 ) adapted to transfer optical pump power from the pump unit ( 2 ) to the active fiber ( 1 ). Moreover, in a safety state, the pump unit ( 2 ) is adapted to give a pulsed pump power whose mean power is lower than a prescribed safety limit.  
     A method of preventing emission of optical power exceeding a prescribed safety limit on interruption of an optical fiber ( 3 ) which transfers pump power from a pump unit ( 2 ) to an active fiber ( 1 ), comprises changing the mean power of the pump power in response to a signal received from the active fiber ( 1 ) so that the mean power assumes a value below said safety limit if said signal is not received, and assumes a nominal value if said signal is received.

RELATED APPLICATION

[0001] This application is a continuation of U.S. Ser. No. 09/308,342filed Nov. 14, 1997 now U.S. Pat. No. ______, which claims the priorityof PCT/DK97/00518 filed Nov. 14, 1997.

TECHNICAL FIELD OF THE INVENTION

[0002] The invention relates to an optical amplifier comprising anactive fiber, a pump unit spaced from the active fiber and adapted togive a nominal, continuous pump power in an operational state, and apump fiber adapted to transfer pump power from the pump unit to theactive fiber.

[0003] The invention moreover relates to a method of preventing emissionof optical power exceeding a prescribed safety limit upon interruptionof an optical fiber which transfers pump power from a pump unit to anactive fiber.

BACKGROUND OF THE INVENTION

[0004] Optical fiber amplifiers for amplifying optical signals typicallyconsist of a length of active fiber, which may e.g. be an erbium-dopedfiber, and a unit for generating pump power, e.g. a pump laser. When theactive fiber is pumped with a strong optical signal (the pump signal)having a wavelength range different from that of the signal to beamplified, and a communications signal is launched into the amplifier, asignal coherent with the signal on the input will occur on the output ofthe active fiber. The gain is determined i.e. by the power of the pumpsignal.

[0005] The active fiber may be arranged at a considerable distance (e.g.10-50 km) from the pump laser, in which case the amplifiers are referredto as remote-pumped amplifiers. With e.g. remote-pumped preamplifiers,also called RILP (Remote In-Line Preamplifier), the active fiber is thusspaced from the actual receiver of the optical signals, and it is pumpedfrom the receiver. This takes place via an optical fiber, typically, butnot necessarily, the same fiber as transmits the communications signalsfrom the active fiber to the receiver.

[0006] The light transmitted in such fibers, in the form ofcommunications signals or pump power, is typically harmful to the humaneye. Therefore, because of situations with access to fiber ends ornon-connected connectors, it is prescribed by various standards how muchoptical power may be transmitted from an open fiber end in thesesituations. These situations may e.g. occur in case of repair,maintenance and testing of systems, or when a fiber has broken, or aconnector is disassembled. It is the temporal mean power of the lightthat is harmful to the eye.

[0007] To achieve the desired function of a remote-pumped amplifier, itis necessary to emit levels of pump power in the fiber from the pumplaser which significantly exceed the mentioned safety limits. To complywith the safety standards, it is therefore necessary to reduce the pumppower in the event that the fiber transmitting the pump power isinterrupted between the pump laser and the active fiber.

[0008] Further various communications equipment standards prescribe thatthe equipment must be capable of automatically resuming normal operationwhen the transmission path has been re-established after a break andtransmission signals are transmitted again. For remote-pumpedamplifiers, such as e.g. RILP, this requirement, however, is not easy tosatisfy, as the reduced pump power results in a considerable reductionin the gain of the active fiber. Therefore, the communications signalsarriving at the receiver after the re-establishment of the transmissionpath, will frequently be below the sensitivity limit of the receiverbecause of the reduced pump power.

[0009] This problem has previously been solved e.g. by using anadditional fiber from the receiver to the active fiber. This fiber, incombination with the transmission fiber, is used for passing a controlsignal from the receiver to the active fiber and back to the receiver.When the control signal is present, there is no break on the fiber andconsequently no access to the strong optical pump power, and the pumplaser can therefore pump with full power. When, on the other hand, thecontrol signal is absent, this indicates a break on the fiber, involvingthe risk that the optical power hits an eye, and the pump power istherefore reduced to a safe level until the control signal is presentagain.

[0010] Although this solution is technically adequate, it is vitiated bythe serious drawback that it requires an additional fiber typically of alength of 10-50 km. Moreover, a detector or a coupler capable ofreturning the control signal to the receiver must necessarily beprovided at the active fiber.

[0011] Systems which are able to reduce the optical output power from afiber amplifier in case of a broken fiber are also known. These systemsdo not involve remote-pumped amplifiers and, therefore, they only reducepower of the communications signals because the pump power never leavesthe fiber amplifier itself.

[0012] Such a system is described in DE 42 22 270 in which the pumppower to the active fiber is reduced if an alarm signal is received fromthe receiver in the other end of a transmission fiber, said alarm signalindicating that the communications signals are not received, e.g.because of a broken fiber. However, this can only be done if there is anextra fiber or another transmission channel for transfer of the alarmsignal and, therefore, this system also has the above-mentioneddrawback. Further, the system is not suitable for reducing pump power,unless a special detector unit as above is provided at the active fiberfor generation of an alarm signal.

[0013] A similar system is known from U.S. Pat. No. 5,428,471 in whichtwo parallel fibers are used for transmission in respective directions.When a fiber amplifier in one direction detects an absent input signal amessage is sent via the opposite fiber back to the previous fiberamplifier to reduce or shut down its optical power level. Therefore,also this system has the above-mentioned drawbacks.

SUMMARY OF THE INVENTION

[0014] The invention provides an optical amplifier of the stated typewhich, in case of a pump fiber break, is capable of complying with thestandards of how much light may be transmitted on the fiber, and whichis simultaneously capable of returning to full pump power when the fiberconnection has been re-established. This may take place by using theexisting fiber or fibers, which means that no additional fiber isrequired exclusively for this purpose, and that an additional detectoror coupler at the active fiber is obviated.

[0015] This is achieved according to the invention in that, in a safetystate, the pump unit is moreover adapted to give a pulsed pump powerwhose mean power is lower than a prescribed safety limit.

[0016] Pulsing of the pump power ensures that its mean power can be keptso low in the safety state that the emitted light is unharmful to thehuman eye, while the instantaneous power of the pulses is sufficientlyhigh for the active fiber to respond on reception of these pulses and toinform the pump unit—via the pump fiber or optionally another existingfiber—that the pump fiber is now intact again. When—and if—a pump pulsearrives at the active fiber, the optical power contained in the pulsewill be absorbed by the active fiber which, in response to the pulse,simultaneously generates a spontaneous noise called ASE (AmplifiedSpontaneous Emission), and this ASE signal may then be returned to thepump unit.

[0017] The pump unit, which generates the required pump power, may beconstructed in different ways. In an expedient embodiment a pump laseris used.

[0018] When the pump unit is adapted to detect whether an optical signalis returned from the active fiber in response to the pulsed pump power,it is ensured that the pump unit can switch between the operationalstate and the safety state in dependence on the returned signal.

[0019] When the pump unit is adapted to generate the pulsed pump poweras pulses repeated with a given frequency, it is ensured that also thereturned ASE noise, in the situation where the pulses arrive at theactive fiber, will have this frequency, a corresponding ASE pulse beingreturned for each emitted pulse. Therefore, the pump unit mayexpediently be adapted to perform the detection of whether an opticalsignal is returned from the active fiber in response to the pulsed pumppower, by detecting whether an optical signal with the given pulsationfrequency is received.

[0020] Then, the pump unit may be adapted to remain in the safety stateif it is detected that no optical signal is returned from the activefiber in response to the pulsed pump power, and to switch to theoperational state if it is detected that such a signal is returned.

[0021] Switching from the safety state to the operational state may takeplace via an intermediate state in which the pump unit can give acontinuous pump power superimposed by a pulsed signal. This ensures thatin this intermediate state the active fiber may be given a sufficientpump power for it to operate practically normally and therefore toamplify any communications signals, while enabling it to be controlledby means of the pulses whether the connection is still intact untilcommunications signals proper are received. Expediently, as stated inclaim 9, the superimposed pulsed signal in the intermediate state mayhave the same shape as the pulsed pump power in the safety state. As aresult, the same detector circuit may be used in the two states.

[0022] It will therefore be expedient that optical information signalsmay moreover be transferred from the active fiber to the pump unit, andthat the pump unit comprises means for detecting whether suchinformation signals are received.

[0023] A particularly expedient embodiment is obtained when the saidoptical information signals are transferred from the active fiber to thepump unit via the pump fiber, as the system then just needs one fibercapable of serving as a transmission fiber and pump fiber, and moreovercapable, in the safety state, of transferring the pulsed pump power andthe possible response to this.

[0024] When the pump unit is in the operational state, it may be adaptedto remain in this state as long as information signals are received, andto switch to the safety state if no information signals are received.

[0025] When the pump unit is in the safety state, it may be adapted toremain in this state if no returned optical signal in response to thepulsed pump power is detected, and to switch to the intermediate stateif such a signal is detected.

[0026] When the pump unit is in the intermediate state, it may beadapted to switch to the operational state if information signals arereceived, to switch to the dwell state if no returned optical signal inresponse to the pulsed pump power is detected, and to remain in theintermediate state if a returned optical signal in response to thepulsed pump power is detected and no information signals are received.

[0027] Finally, the pump unit may be adapted to inhibit the detection ofwhether an optical signal in response to the pulsed pump power isreturned from the active fiber, until a selected period of time haselapsed after the transmission of each pulse from the pump unit. Thisensures that the detector circuit ignores the reflections that will bereturned from the pump fiber, irrespective of whether this is intact orbroken, and instead exclusively detects the ASE noise which can onlyoriginate from the active fiber, and which will last considerably longerthan the reflections from the pump fiber.

[0028] As mentioned, the invention also relates to a method ofpreventing emission of optical power exceeding a prescribed safety limiton interruption of an optical fiber which transfers pump power from apump unit to an active fiber. When the mean power of the pump power ischanged in response to a signal received from the active fiber such thatthe mean power assumes a value below said safety limit if said signal isnot received, and assumes a nominal value if said signal is received, itis ensured that the mean power may automatically be reduced to a safelevel when a break occurs on the optical fiber.

[0029] This may expediently take place in that the mean power below saidsafety limit is generated by pulsing the pump power with a givenfrequency, and that the signal received from the active fiber isdetected by detecting whether a signal with the given pulsationfrequency is received.

BRIEF DESCRIPTION OF THE DRAWINGS

[0030] For a more complete understanding of the present invention andthe advantages thereof, reference is now made to the followingdescription taken in conjunction with the accompanying drawings, whereinlike reference numbers represent like parts, in which:

[0031]FIG. 1 shows an example of a remote-pumped optical amplifierconsisting of an erbium-doped fiber, a receiver and pump unit and atransmission and pump fiber;

[0032]FIG. 2 shows the receiver and pump unit of FIG. 1 in greaterdetail;

[0033] FIGS. 3A-3D show curve shapes of signals in the receiver and pumpunit when this is in a safety state, and

[0034]FIG. 4 shows curve shape of a pump signal in the receiver and pumpunit when this is in an intermediate state.

DETAILED DESCRIPTION OF THE INVENTION

[0035]FIG. 1 shows an example of an optical remote-pumped amplifier ofthe invention. The example involves an optical preamplifier consistingof an erbium-doped fiber 1 and a receiver and pump unit 2 connected tothe erbium-doped fiber by a transmission and pump fiber 3, which maytypically have a length of 10-50 km.

[0036] In the receiver and pump unit 2, the light arriving from thefiber 3 passes via a wavelength multiplexer 4 to a receiver or amplifiercircuit 5, in which the transmission or information signals contained inthe light may be received and optionally be passed on for furtherprocessing. A pump laser 6 generates optical pump power which istransmitted via the wavelength multiplexer 4 out on the fiber 3 in adirection toward the erbium-doped fiber 1. Typically, the light with theinformation signals may have a wavelength of 1550 nm, while oppositelydirected pump light may have a wavelength of 1480 nm, thereby enablingthe wavelength multiplexer 4 to transmit the information signals fromthe fiber 3 to the receiver circuit 5 and the pump power from the pumplaser 6 to the fiber 3.

[0037] The mean power of the pump signal will usually be considerablyhigher than the mean power of the transmission signals, and interruptionof the fiber 3 between the unit 2 and the erbium-doped fiber 1 wouldtherefore involve the risk that a harmful quantity of light might hit aneye if no safety measures were taken.

[0038] The receiver and pump unit 2 is therefore adapted to be able toassume three states.

[0039] In an operational state assumed when the receiver 5 detects acommunications signal, the pump laser gives full nominal pump power, asthe received communications signal is a guarantee that the fiber 3 isintact all the way to the erbium-doped fiber 1.

[0040] In a safety state assumed when there is no connection from thereceiver and pump unit 2 to the erbium-doped fiber 1, the pump signal ispulsed so that its mean power is below 10 mW, which means i.a. that theequipment may be categorized as safety class 1 according to the IEC 825recommendations.

[0041] An intermediate state is assumed when connection to anerbium-doped fiber is detected, while the receiver 5 has not yetdetected a communication signal. In this state, the pump power isdetected so that the mean power constitutes about ⅔ of the nominal pumppower.

[0042] If the fiber 3 is intact when the system is in the safety stateor the intermediate state, the pump pulses will reach the erbium-dopedfiber 1, and the optical power contained in the pulses will be absorbedby the erbium-doped fiber, while a spontaneous noise pulse of so-calledASE noise (Amplified Spontaneous Emission) is generated in response toeach pulse in the erbium-doped fiber 1. These ASE noise pulses will thenbe returned via the fiber 3 to the unit 2, where, as will be describedmore fully below, they can be detected to indicate that there is nobreak on the fiber 3.

[0043] If, on the other hand, there is a break on the fiber 3, thepulses emitted from the pump laser 6 will not reach the erbium-dopedfiber 1, and thus no ASE noise pulses will be generated.

[0044] When the system is started, the receiver and pump unit 2 willfirst assume the safety state, while it is checked whether connection toan erbium-doped fiber has been established. When this has been found tobe the case, the unit 2 switches to the intermediate state until thereceiver 5 detects a transmission signal. In the intermediate statewhere the pump power is about ⅔ of the nominal value, the transmissionquality is just slightly inferior relative to normal function, and therest of the system is therefore capable of performing a normal start-upprocedure. When the receiver 5 detects a transmission signal, the unit 2switches to the normal operational state.

[0045] If it is detected at any time while the system is in theoperational state that the receiver 5 no longer detects a communicationssignal, the unit 2 immediately switches to the safety state, as themissing communications signal may, for example, be caused by a fiberbreak between the unit 2 and the erbium-doped fiber FIG. 2 shows ingreater detail how the receiver and pump unit 2 may be constructed. Aswill be seen, the pump laser 6 is controllable partly from a controlunit 16 and partly from a clock generator 17. The control unit 16decides which of the three above-mentioned states the unit is to assume,while the clock generator 17 determines the pulse frequency in thestates where the pump laser is pulsed. The pulse frequency may, forexample, be selected at 75 Hz.

[0046] Having passed the wavelength multiplexer 4, the light receivedfrom the fiber 3 may optionally be amplified in an optical amplifier 7,following which it is split into two branches in the optical coupler 8.The branch having the units 12-15, which will be described more fullybelow, detects whether the received light includes ASE noise pulses withthe pulse frequency, while the detector 11 detects whether the lightcontains communications signals.

[0047] In the operational state, the pump laser 6 pumps continuouslywith the nominal pump power, and the communications signals receivedfrom the fiber 3 reach the detector 11 via the wavelength multiplexer 4,the amplifier 7 and the coupler 8. The detector 11 passes the signals onfor further processing and also informs the control unit 16 thatcommunications signals are received at the moment. The control unit 16therefore ensures that the laser 6 continues to give full pump power.

[0048] If the detector 11 detects that the communications signals failto appear, it informs the control unit 16 which immediately sets thepump laser 6 in the safety state via the connector 9, where the pumppower is transmitted in pulses determined by the clock generator 17. Thepulsed pump power may, for example, look as shown on curve A in FIG. 3.The repetition frequency of the pulses is here selected to be 75 Hz, andthe duty cycle is selected such that the resulting mean power is below10 mW. Typically, the nominal power will be 110 mW, and the duty cyclewill then be {fraction (1/11)} or less.

[0049] When the fiber 3 is intact, the pulses will move along it untilthey reach the erbium-doped fiber 1, and part of the pulse will bereflected on the way because of Rayleigh scattering, and, therefore, areflected signal will return to the receiver and pump unit 2 from thefiber 3. This signal may, for example. look as shown on curve B in FIG.3. It is noted that the amplitude of the reflected signal isconsiderably smaller than the emitted pulses. When the pump pulsereaches the erbium-doped fiber 1, this will be active and startgenerating ASE noise, which is likewise passed via the fiber 3 back tothe receiver and pump unit 2. The ASE noise will be generated as long asthe pulse lasts, and will then decrease according to an exponentialcurve whose time constant is long with respect to the transmission timeon the fiber 3. The ASE noise received on the receiver and pump unit 2may look as shown on curve C in FIG. 3. These ASE noise pulses are usedin the receiver and pump unit 2 as an indication that the fiber 3 isintact.

[0050] Since, however, the signal received from the fiber 3 is the sumof curves B and C, the inhibition circuit 12 provides for blocking ofthe received signal as long as the signal reflected from the fiber 3lasts (curve B) Because of the pulse transit time in the fiber, thiswill be a period after the end of the transmitted pump pulse, which willbe about 0.5 msec. with a fiber length of 50 km. As mentioned below, areflected signal will return also if the fiber is broken, but also thissignal will at most be of the same duration. The inhibition circuit 12is also controlled by the clock generator 17. Thus, only the exponential“tail” of the ASE noise pulse will be present on the output of theinhibition circuit 12, as shown on curve D in FIG. 3.

[0051] This signal, like the emitted pulses, has a repetition frequencyof 75 Hz, and it is now passed through a bandpass filter having a centerfrequency of the 75 Hz and a bandwidth of e.g. 15 Hz to filter outpartly components from a possible communications signal partly signalsoriginating from a constant spontaneous emission in the erbium-dopedfiber 1.

[0052] The bandpass-filtered signal is then fed to a sample-and-holdcircuit 14 which samples with the same frequency as the pulsed pumpsignal so as to provide a sampling value for each pulse. The sampledvalues are lowpass-filtered in the lowpass filter 15 and are thencompared in the control unit 16 with a threshold value to decide whethera sufficiently great value of the ASE noise is received. If the controlunit 16 detects that the ASE noise pulses are above the threshold value,it instructs the pump laser to switch to the intermediate state, whichwill be described below, as the fiber 3 must be intact.

[0053] If, on the other hand, the fiber 3 is broken, no ASE pulses cancome from the erbium-doped fiber 1, as the pump pulses do no reach it.But then there will be a strong reflection of the emitted pulse from thebreak. Depending on the distance from the break, this reflection willusually have a considerably greater amplitude than both curves B and Cin FIG. 3; but this reflection will be over at the latest simultaneouslywith curve B and will therefore be blocked by the inhibition circuit 12,so that the control unit 16 does not detect any signal. This is anindication of a break on the fiber, and the control unit thereforeinforms the pump laser 6 to remain in the safety state.

[0054] When the control unit 16 has established that ASE pulses return,the unit switches to the intermediate state, as mentioned, where thepump laser emits a signal, as shown in FIG. 4. The power level betweenthe pulses is selected at about ⅔ of the nominal pump power, and thepeak level of the pulses corresponds to the nominal power.

[0055] If the fiber 3 is still intact, a signal corresponding completelyto the one described above and shown in FIG. 3 will be returned, theamplitude of the signals being merely smaller. The difference is justthat the erbium-doped fiber 1 will now receive sufficient pump power tomake it capable of passing on communications signals. When these aredetected by the detector 11, the control unit switches to the normaloperational state. The intermediate state is necessary, because theoperational state can only be maintained when communications signals arereceived. Therefore, in this circuit, it will not be expedient to switchdirectly from the safety state to the operational state.

[0056] If the ASE pulses disappear in the intermediate state, thisindicates that the fiber has been interrupted again, and the controlunit 16 will therefore return to the safety state.

[0057] The repetition frequency of the emitted pulses is here selectedat 75 Hz; but may also assume other values of course. It must besufficiently low so that the next ASE pulse is not emitted before theASE pulse caused by the pulse has died away, and the lower limit of thefrequency is determined by the maximum time it may take the system toswitch from the intermediate state to the safety state.

What is claimed is:
 1. An optical amplifier, comprising: a receiver andpump unit operable to receive optical signals from a fiber optic link,the receiver and pump unit operable to convert the optical signals intocommunication information, the receiver and pump unit operable totransmit the communication information, the receiver and pump unitoperable to provide optical pump power to the fiber optic link, thereceiver and pump unit operable to operate in a safety state with theoptical pump power having a mean power lower than a desired safetythreshold.
 2. The optical amplifier of claim 1 , wherein the receiverand pump unit includes a pump laser for generating the optical pumppower.
 3. The optical amplifier of claim 1 , wherein the receiver andpump unit is operable to detect whether an optical signal is returned inresponse to the optical pump power.
 4. The optical amplifier of claim 1, wherein the receiver and pump unit is operable to generate the opticalpump power as pulses which are repeated with a given frequency.
 5. Theoptical amplifier of claim 4 , wherein the receiver and pump unit isoperable to detect whether an optical signal having the given frequencyis returned from the fiber optic link in response to the optical pumppower.
 6. The optical amplifier of claim 3 , wherein the receiver andpump unit is operable to remain in the safety state if it is detectedthat no optical signal is returned in response to the optical pumppower.
 7. The optical amplifier of claim 3 , wherein the receiver andpump unit is operable to switch to an operational state if it isdetected that an optical signal is returned in response to the opticalpump power.
 8. The optical amplifier of claim 7 , wherein the receiverand pump unit is operable to switch from the safety state to theoperational state via an intermediate state, the receiver and pump unitoperable to give a continuous optical pump power super-imposed by aoptical signal in the intermediate state.
 9. The optical amplifier ofclaim 8 , wherein the super-imposed optical signal in the intermediatestate has the same shape as the optical pump power in the safety state.10. The optical amplifier of claim 7 , wherein the receiver and pumpunit remains in the operational state as long as optical signals arereceived.
 11. The optical amplifier of claim 10 , wherein the receiverand pump unit is operable to switch to the safety state if no opticalsignals are received.
 12. The optical amplifier of claim 3 , wherein thereceiver and pump unit is operable to switch to an intermediate state inresponse to detection of an optical signal.
 13. The optical amplifier ofclaim 12 , wherein the receiver and pump unit, when in the intermediatestate is operable to switch to the operational state if optical signalsare received carrying communication information.
 14. The opticalamplifier of claim 13 , wherein the receiver and pump unit is operableto switch to the safety state if no optical signals are received. 15.The optical amplifier of claim 13 , wherein the receiver and pump unitis operable to switch to the intermediate state in response to receivingoptical signals that do not carry communication information.
 16. Theoptical amplifier of claim 3 , wherein the receiver and pump unit isoperable to inhibit detection of whether an optical signal is returnedin response to the optical power until a selected period of time haselapsed after the emission of each pulse of the optical pump power fromthe receiver and pump unit.
 17. A method of controlling power on anoptical fiber, comprising: generating an optical pump power;transmitting the optical pump power; determining whether an opticalsignal has been received; lowering a mean power of the optical pumppower below a desired safety level in response to no optical signalbeing received.
 18. The method of claim 17 , further comprising:increasing the mean power of the optical pump power to a subnominallevel in response to receipt of the optical signal.
 19. The method ofclaim 18 , further comprising: determining whether the optical signalcarries communication information; increasing the mean power of theoptical pump power to a nominal level in response to receipt ofcommunication information.
 20. An optical amplifier, comprising: meansfor generating an optical pump power; means for transmitting the opticalpump power; means for determining whether an optical signal has beenreceived; means for lowering a mean power of the optical pump below adesired safety level in response to no optical signal being received.